In the area of linear circuit design (also known as "analog" circuit design), PNP transistors must inevitably be used along with their NPN counterparts. While NPN operating characteristics have been optimized, little effort has been provided to maximizing the operating characteristics of the PNP devices. As a result, the overall performance of the circuit is limited by the PNP behavior.
CMOS technology is generally a large part of any system. The effort to design large analog systems on a single chip requires front-end processes which can provide the needed high performance components. In previous systems, different technologies could be optimized on separate chips. However, when the entire system is integrated on one chip, the processes must be designed such that each technology is optimized.
Heretofore, the PNP transistor technology has been compromised in order to benefit the NPN and CMOS transistors. The two types of PNP devices available in most front-end processes for a merged bipolar and CMOS technology are the lateral PNP and the substrate PNP. These structures are deficient in many respects. First, they exhibit a low cut-off frequency due to their wide base regions. Second, the devices exhibit current gain roll-off at low to moderate current levels, due to the use of lightly-doped base regions. Third, the substrate PNP is not an isolated device, since it is configured such that the collector of each PNP transistor is the substrate. Thus, all substrate PNP devices on a single chip have a common collector. Fourth, the lateral PNP device requires a large amount of area in order to build the isolated structure. Fifth, both devices have relatively high series resistance in their collectors.
Therefore, a need has arisen in the industry for a technology which incorporates NPN transistors, CMOS technology, and high performance PNP transistors onto a single chip, without sacrificing the operating characteristics of the PNP device or unduly increasing the complexity of the manufacturing process. The PNP transistor should possess a high cut-off frequency, moderate current gain, isolated structure, and low series resistance.